The present invention relates to boiler body structure of water-tube boilers such as once-through boilers, natural circulation water-tube boilers and forced circulation water-tube boilers.
As a body structure of water-tube boilers, there has been provided, hitherto, one in which a plurality of water tubes are arranged into an annular shape, thereby forming an inner water tube array, where a combustion chamber is defined by a space inner than the inner water tube array, while a plurality of water tubes are further arranged into an annular shape outside the inner water tube array, thereby forming an outer water tube array, where a gas flow passage is defined between the two water tube arrays. Heat transfer primarily by radiation is performed within the combustion chamber, and heat transfer primarily by convection is performed in the gas flow passage.
In this water-tube boiler, for improvement in boiler efficiency, heat transfer fins are provided on the water tubes as a measure for increasing the heat transfer area. More specifically, fully circumferential fins are provided on a specified number of outer water tubes placed near an opening formed in the outer water tube array so that the boiler efficiency can be improved (e.g., see Japanese Patent Laid-Open Gazette Hei 9-133301 (Patent Abstracts of Japan/Publication Number: 09133301A)). However, out of the heat transfer surfaces confronting the gas flow passage, it is only part of heat transfer surfaces of the outer water tube array that are improved in heat transfer surface structure. That is, the heat transfer surface structure is no more than set in two stages, the vicinities of the opening in the outer water tube array and the upstream side of the opening. Still, whereas the water tubes equipped with fully circumferential fins are provided in a region where the gas temperature is lower than a specified temperature in order to prevent the fully circumferential fins from burning out, this region is a much limited region on the downstream side out of the whole gas flow passage. Therefore, the water-tube boiler is other than designed so as to attain enough increase in heat transfer amount. Moreover, although some extent of increase in heat transfer amount can be attained by the provision of the fully circumferential fins, yet there is a need for further device in order to make effective use of the whole heat transfer surfaces of the water tubes equipped with the fully circumferential fins.